Highly linear and stable instrumentation amplifiers are required in certain applications such as the amplification of signals from thermocouples, strain gauges, and thermistors.
Present day amplifiers have a preset gain which varies with temperature on the order of four or five parts per million per degree Celsius (ppm/.degree. C). To provide some perspective as to the significance of a drift of 4 ppm/.degree. C., in a 12-bit digital system, one half of the least significant bit is equivalent to 128 ppm, and in a 16-bit digital system, one half of the least significant bit is equivalent to 8 ppm. An amplifier with a 4 ppm/.degree. C. which operates in a temperature range of 0.degree. to 70.degree. C. can have a drift of 280 ppm over the temperature range.
Such instrumentation amplifiers generally consist of operational amplifiers and resistors. Since the gains of operational amplifiers available today is in excess of a million, the predominant drift producing mechanisms are the gain setting resistors.
Therefore, it can be appreciated that a resistor structure which provides significantly less drift in an instrumentation amplifier type of circuit is highly desirable.